As far as I know she was painted up for a special charter. Some kind of specialised “off the beaten track” kind of holiday/tour. TCS Expeditions do a round the world trip which takes in the Great Barrier Reef as one of the stops. Which may explain the Brisbane sighting.
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On their website they claim to have their own private 757 “maintained” by Air 2000. I reckon they just charter it though ?
If you’re interested in taking the tour, you won’t get much change out of $45,000 USD
Here is G-OOOX in Cambodia as part of the world tour.